1. Field of the Invention
This invention relates to adjustable die face pelletizers and specifically to a face-cutting apparatus for making pellets of thermoplastic material while controlling extrusion back pressure in the thermoplastic extruder.
2. Description of the Prior Art
A face-cutting apparatus of the type described is disclosed in U.S. Pat. No. 3,973,890. In such an apparatus, the die plate means associated with the thermoplastic extruder is hollow and cylindrically shaped and is provided with radially extending extrusion orifices through which heat-plastified material or extrudate is extruded by reason of the operation of a rotating feed screw which pressurizes the material upstream of the die plate means. The die plate means comprises a pair of nested die plates mounted for slight selective positioning for altering or adjusting the degree of alignment or misalignment of the orifices in the nested die plates relative to one another.
The cutter in such a face-cutting apparatus has a plurality of blades projecting radially from a cylindrical body and extending longitudinally therealong. At the free end of each blade is a cutting edge that is operatively associated with the downstream side of the die plate means, so that upon rotation of the cutter, heat-plastified material extruded through the orifices of the die plate means is sheared into pellets. Adjacent pairs of blades on the cutter and the intervening body portion establish continuous longitudinally extending channels into which the newly sheared pellets collect. A pellet treatment fluid is flushed through the channels formed by the cutter and the die plate means in order to sweep the pellets from the cutting edge of the blade as they are severed. The pellets are then collected at the downstream end of the cutter.
In the prior art devices the die plate means are rotatable with respect to one another. In one relative position, the orifices in the plates are completely aligned to offer minimum resistance to the extrusion of heat-plastified material through the orifices. By changing the relative position of the two die plates, the resultant misalignment of the orifices introduces an obstruction at the interface between the plates to the extrusion of material. Selective changes in the relative rotational position of the nested die plates, in response to changes in the pressure of the heat-plastified material upstream of the die plates, maintains a substantially constant rate of extrusion. Furthermore, because the flow obstruction occurs at the interface between the die plates, the cross-section of the material at the downstream face of the die plate closest to the cutter will be constant. Since the rate of extrusion is kept constant, the thickness of the pellets is uniform.
These prior art devices for maintaining extrusion flow rate by adjusting extrusion back pressure presented several disadvantages.
First, the die plate means have a relatively short operating life. Thus, it is necessary to frequently shut down the thermoplastic extruders in order to replace worn die plates. Such a replacement operation becomes both time consuming and costly over the operating life of an extruder.
Secondly, it is desirable for an extruder to have the ability to produce thermoplastic pellets of varying diameters for different applications. In the prior art devices, the die plate means only had one set of extrusion orifices all having the same diameter. Hence, the prior art devices had no capability of producing more than one size of thermoplastic pellet. Instead, it was necessary to stop the machine, clean it and then replace the die plates with another set of die plates having different size orifices. As was mentioned above, this process is both time consuming and costly.
Thus, there has been a need in the art for a die plate means to be used in conjunction with a face-cutting apparatus having a substantially longer life than the prior art devices.
Furthermore, there has been a need in the art for such a long-life device which has the ability to produce different sizes of thermoplastic pellets without costly shutdown and cleaning procedures.
Still further, there has been a need in the art for a die plate means able to quickly and easily convert from the production of one size (diameter) pellet to another size pellet without readjusting the relative rotational positions of the nested die plates.
These and other objects are provided by the apparatuses and methods of the present invention.